1. Field of the Invention
The invention relates to a method and to an apparatus for measuring spacings between optical surfaces of single-lens or multi-lens optical systems. Spacings of such a type are generally specified with reference to the optical axis of the optical system.
2. Description of the Prior Art
For the purpose of measuring spacings between optical surfaces of a single-lens or multi-lens optical system it is known to use short-coherence interferometers such as are described, for instance, in FR 2 803 027 A1 and in a paper from R. Wilhelm et al. entitled “On-axis, non-contact measurement of glass thicknesses and airgaps in optical systems with submicron accuracy”, Proc. of SPIE, Vol. 6616 (2007), 66163P-1 to 66163P-12. Interferometers of such a type contain a light-source that generates measuring light with a very short coherence length. The beam path of the measuring light is split up with the aid of a beam-splitter into a reference arm and a measuring arm. The measuring light conducted in the measuring arm is directed onto the optical system while the optical path length of the measuring light conducted in the reference arm is varied with the aid of a mobile mirror array or similar. Measuring light reflected on the optical surfaces of the optical system is superimposed on a photodetector with the measuring light conducted in the reference arm. From interference phenomena detected by the photodetector, differences in the optical path length can be inferred that the measuring light travels on its path between the optical surfaces. For in consequence of the short coherence length of the measuring light that is used, interference phenomena arise at the photodetector only when the optical path lengths in the reference arm and in the measuring arm correspond.
Measuring apparatuses that make use of this measuring principle are offered for sale by, for example, FOGALE nanotech, Nîmes, France.
In connection with the use of short-coherence interferometers for measuring spacings between optical surfaces of optical systems it is, however, necessary that the measuring light impinges onto the optical surfaces as perpendicularly as possible. Even in the case of slightly tilted surfaces, so little light is reflected back into the interferometer that no interference signals or at best—on account of the then very low signal-to-noise ratio—barely detectable interference signals arise at the detector. Even when in the case of tilted optical surfaces the interference signals can be detected well, ultimately it is not, as desired, the spacings between the optical surfaces along the optical axis of the optical system that are measured but rather merely the spacings along the measuring direction defined by the measuring-light ray. Since this direction may deviate considerably from the optical axis of the optical system, the measured values acquired in this way are not very meaningful.
For adjusting the optical system the above mentioned paper from R. Wilhelm et al. proposes to first adjust a tip-tilt mount for the optical system using a light ray produced by a laser pointer. To this end a planar mirror is laid on the mount and the light ray is directed on the mirror. The mount is then adjusted until the light ray is reflected back onto itself. In a second step the optical system is laid on the mount and tilted until the light ray impinges perpendicularly on the optical surface pointing towards the interferometer so that it is reflected back onto itself.